Check the url within config.json.
Well, that was easy. Thanks for the quick fix!
I just noticed nodebb forum has no problem in changing the display picture immediately, but on my forum (freshly installed) I have to press ctrl+f5 to clear the cache to make the display picture to appear.
Are you using another cache in front of NodeBB? Like Nginx or CloudFlare?
cloudflare is not a problem because I tested that by disabling it. I use nginx, but I am not sure there is a problem with that.